This invention relates to an automatic gain control (to be referred to as "AGC", hereinafter) circuit for a high-frequency range which includes VHF (very high frequency) and UHF (ultra high frequency), and more particularly to a high-frequency AGC circuit having a constant impedance characteristics and a broad controllable gain range at a high gain level.
Conventionally, two kinds of AGC circuits have have been used for the VHF and UHF frequency ranges, namely: the so-called "variable emitter current type AGC circuit" which controls an amplifier gain by regulating the current transfer factor of a transistor through varying the bias current of the transistor; and an AGC circuit for an amplifier having two cascaded amplifying stages and a variable attenuator, which attenuator is formed of a diode connected between the two amplifying stages and a load resistor or a terminating resistor, whereby the gain of the amplifier is controlled by regulating the attenuation of said attenuator through changing the equivalent resistance of the aforesaid diode by modifying its forward bias current.
The variable emitter current type AGC circuit has a shortcoming in that, when the emitter current is varied in excess of a certain limit, the amplifier output is considerably distorted because the high-frequency amplification of the transistor depends on the emitter current, so that even a signal of comparatively low level generates higher harmonics components, which harmonics generation prohibits the use of the AGC circuit at a high signal level and restricts the variable gain to a comparatively narrow range. The conventional AGC circuit using the attenuator has the shortcomings that non-linear portions of various operative characteristics of the attenuator diode tend to cause harmonics distortion and deterioration of the overall amplifying characteristics of the amplifier, that there are certain limitations on the bias current, and that the resistance loss within the diode is not negligible even when the equivalent resistance of the diode is at its minimum.
With the aforesaid conventional AGC circuits, input and/or output impedance of the amplifier changes as the gain or attenuation level of the amplifier varies, so that signal reflection may be caused at the boundaries between the amplifier and outside circuits connected to its input and output terminals. To eliminate such reflection, buffer circuits are often used at the input and output sides of the amplifier.